Re: [boost] PP re-entrancy question

From: Paul Mensonides (pmenso57_at_[hidden])
Date: 2005-12-29 21:19:30

> -----Original Message-----
> From: boost-bounces_at_[hidden] 
> [mailto:boost-bounces_at_[hidden]] On Behalf Of Arkadiy Vertleyb

> Hi,
> 
> The typeof library defines a macro, REGISTER_TEMPLATE, that 
> is intended for use by the library users.  Once people 
> started using the library, they eventually wanted to use this 
> macro inside the Boost PP looping constructs, such as 
> SEQ_FOR_EACH, etc.
> 
> Since the implementation of  REGISTER_TEMPLATE itself uses 
> quite a few looping constructs, people ran into re-entrancy issues.

Which looping constructs does it use?

> I would like the users to be able to use the macro with 
> whichever looping construct they prefer, and, AFAIU, the 
> easiast (only?) way to achieve this would be to limit the 
> macro implementation to only use automatically re-entrant 
> macros, such as BOOST_PP_REPEAT.

That depends.  If you can (re)specify the implementation in terms of only one
"recursion state" (such as 'z' for REPEAT), then it is best to provide both:

#define REGISTER(...) REGISTER_Z(DEDUCE_Z(), ...)
#define REGISTER_Z(z, ...) // ...

If the implementation requires (or is significantly better using) multiple
states, then, yes, you should have it do everything automatically.

> Is this the right way to go for this particular case, and 
> generally for any macro that is in the library interface?

That's the general answer.  The pp-lib fails to provide a unifying way to deal
with recursion, which is why these problems pop up from time to time.

Regards,
Paul Mensonides